Frequently, several types of test instruments are connected together to create a test system for verifying the functions of an electrical device. Each instrument employed in a test system is typically connected with other test instruments and a device under test (DUT) by cables and switches. Each test instrument, cable and switch in the test system has a unique signal path that may affect the operation and measurements obtained by the test system. Furthermore, test systems often employ signal conditioning hardware, e.g. amplifiers and signal splitters that further impact the test system results.
In general, test instruments in test systems may be calibrated with respect to what the test instrument measures; however, the signal connections in the test instrument will always have some effect on the measurement being made. To obtain the most accurate measurements it is typically necessary to calibrate the test system as a whole rather than each individual test instrument. To calibrate the test system as a whole, the measuring effects of each element in the signal path should be determined and subtracted from the raw measurement data. For instance, a cable may be used to connect the test system to the DUT. If that cable has a signal loss of 1 dB, it will affect the measurement data by 1 dB. If the loss of the cable is known in advance, then 1 dB may be added to the measured data to correct for the cable loss.
Prior attempts at test system calibration typically depend on the topology of the test system, and rely on a fixed signal path. In that case, equations may be written that compensate for each element in the signal path. The individual elements may be characterized in some way, and a fixed set of equations may be utilized to obtain corrected measurement results.
The problem with this approach is that when a test system includes multiple switch paths, or when parts of the signal path change from time to time, or when the overall system topology cannot be determined in advance the characterization of the test system is impractical if not impossible. It is not possible to generate a set of equations to correct the measurements of a test system whose topology is unknown; and if the test system topology is changed, the equations must be reformulated and test software rewritten to compensate for these changes.
Therefore, there is a need for an approach to calibrate a test system as a whole that overcomes the problems and limitations explained above.